1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and more particularly to an image forming apparatus and a cleaning method thereof, wherein the image forming apparatus comprises a plurality of image forming units each forming a toner image of a corresponding color on a photoconductor in accordance with a cleanerless system.
2. Description of the Related Art
There is well known the transfer technology based on corona charging to a photoconductor as a conventional transfer technology for electrophotographic image forming apparatuses. However, this technology generates noxious ozone. To solve this, contact-based transfer technologies were proposed as transfer technologies not generating noxious ozone. For example, Jpn. Pat. Appln. Laid-Open Publication No. 6-110343 discloses the technology that uses a semi-conductive transfer belt and a transfer roller provided at the rear of the transfer belt and applies a transfer bias to the transfer roller to transfer images.
For example, the following systems are applicable to an image apparatus that uses yellow (Y), magenta (M), cyan (C), and black (Bk) toners, forms corresponding toner images, and overlays them to form a color image.
(1) The system that overlays 4-color toner images on one photoconductor and transfers the overlaid toner images onto recording paper at a time.
(2) The transfer drum system that holds recording paper on a transfer drum and forms 4-color toner images by means of four rotations of the transfer drum.
(3) The system that temporarily forms 4-color toner images on an intermediate transferrer and transfers the formed toner images onto recording paper at a time (e.g., Jpn. Pat. Appln. Laid-Open Publication No. 11-249452).
(4) The 4-drum tandem system that arranges four photoconductor drums in parallel and forms 4-color toner images while recording paper passes through these photoconductor drum once (e.g., Jpn. Pat. Appln. Laid-Open Publication No. 2000-155501).
The above-mentioned 4-drum tandem image forming apparatus (example (4) above) forms 4-color toner images while recording paper once passes through respective image forming units including the 4-drum tandem photoconductor drums. It is possible to form a color image in one quarter of the time needed for the other systems that repeat a similar process after completing an image formation process for each color and returning control to the image formation position. Accordingly, the use of this 4-drum tandem system is very advantageous to acceleration of the image formation. The image forming units employ the cleanerless system that removes a cleaning apparatus from the photoconductor drum (e.g., Jpn. Pat. Appln. Laid-Open Publication No. 5-341643). The system contributes to cost saving of the image forming apparatus, life prolongation and miniaturization of the photoconductor, and decrease in the toner consumption.
While the above-mentioned cleanerless 4-drum tandem image forming apparatus is advantageous to the process acceleration, miniaturization, and decrease in the toner consumption, the cleanerless design causes toner remaining on the photoconductor to be adhered to the surface of the photoconductor (filming) and generates white dots on a formed image, thus degrading the image quality. For this reason, the invention disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2000-155501 generates a speed difference between the photoconductor drum and the transfer belt at a cleaning timing to use this frictional force for cleaning. However, this solution complicates operations and makes the settings difficult. A simpler solution is demanded.